1. Field of the Invention
This invention relates to networks of switches operating at layer 2 and executing the spanning tree protocol.
2. Background Information
A longstanding problem in computer network design is the problem of one way loops. A one way loop is formed when a port develops one way communication, the port fails to receive superior BPDUs, and so transitions to designated role and begins forwarding data packets. The ports on each end of the one way link may make this transition, and the port which the fault permits to send packets to the next switch then forms the loop.
The computer network is formed from layer 2 switches with redundant connections, where the redundancy is introduced in order to provide automatic switchover in the event of failure of a switch or link. The layer 2 network having redundant connections is normally prevented from having loops by use of the spanning tree protocol (STP).
The STP selects a “root layer 2 switch” as the root of a spanning tree, and establishes a single link to each switch lower in the spanning tree. One link is established between the root switch and each switch in the first lower layer. Each switch in the first lower layer has one link established to one or more switches in the second lower layer, and each switch in the second lower layer has only one link upstream to one switch in the first lower layer, etc. A logical tree is built, so that communications between a first end station and a second end station is by sending packets from the first end station up the spanning tree to a common switch, and then back down another branch of the tree until the packet reaches the end station to which it is addressed.
Redundant links are eliminated by switching redundant ports into a role known as “blocking”. A port of a switch may be established in one of three roles: “root” role for communications upstream toward the root switch; “designated” role for communications to a switch in the next lower layer in the spanning tree; and, “blocking” role in order to remove the port from communications by the spanning tree protocol in order to eliminate loops in the layer 2 network.
The ports are chosen for the roles of root, designated, or blocking by exchange of STP packets referred to as Bridge Protocol Data Units (BPDU packets). The BPDU packets have an 8 byte field referred to as the “switch ID” and a field referred to as the “port ID”. The BPDUs from one switch and one port are “superior” or “inferior”, depending upon the switch ID field and port ID fields. The switch having the smallest switch ID is chosen as the root switch, and then the switch ID becomes the Root ID. Switches lower in the tree exchange BPDUs, and the switch having the lowest switch ID is superior, and is chosen as the active switch in the logical tree. In the event of redundant ports between a root switch and the next lower switch in the spanning tree, the port having the smallest ID is superior. The superior port is chosen to establish the link. The ports not used to establish a link are transitioned into “blocking” role.
A port is maintained in blocking role by receipt of BPDUs superior to any BPDU which the port could transmit, and the ports each have a timer. If no superior BPDU is received during a timer period, referred to as a BPDU timeout period, the port transitions into designated role. The timer and BPDU timeout period are used to transition the network to a backup switch in the event that a switch being used in the spanning tree develops a fault, and consequently does not transmit its BPDU during the BPDU timeout period. The blocked port then transitions to designated role and begins an active role in the spanning tree, and in some cases triggers a new execution of the STP algorithm.
The one way connectivity problem arises when a particular port develops a faulty receiver, or the port on the other end of a link develops a faulty transmitter. In either case, the particular port receives no packets, and in particular no superior BPDUs. The absence of superior BPDUs causes the particular port to transition into designated port role, and to begin forwarding packets received from structures internal to its switch. The switch at the other end of the link begins receiving the packets and forwarding them, and so creates a loop in the network.
Attempts to solve the one way connectivity loop problem have mainly concentrated on establishing that one way connectivity exists by attempting to detect the absence of packets travelling in one direction along the affected link. These methods are inadequate for reliably detecting one way connectivity loops. These methods are discussed by Radia Perlman in her book Interconnections, Second Edition , published by Addison Wesley, Copyright date 2000, all disclosures of which are incorporated herein by reference, especially at pages 74-75.
There is needed a better method for eliminating loops formed as a result of one way connectivity faults.